MTU

Answered Question
Sep 22nd, 2010
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Hi2All


R1-------------------------------------------------R2

R1 is connected via R2 via Wireless connection, its a pretty old Outdoor Wireless Unit.
How do I check if the default MTU of the Wireless is 1500.


Possible to get MTU information from a PC connection from R1 sending data to a PC2 connection at R2



Best Wishes

Anthony

Correct Answer by Richard Burts about 6 years 7 months ago

Anthony


I do not think that you have yet proved what is the MTU since you have not specified control in the ping about fragmentation. Try pinging this way and see what happens

ping 192.168.1.5 -l 1500 -f


HTH


Rick

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Richard Burts Wed, 09/22/2010 - 09:18
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Anthony


The easy way to do that is to ping from one PC to the other, varying the size of the ping packet and specifying do not fragment. In the Windows ping use the -l parameter to control the packet size and use -f to indicate do not fragment. I would start at 1500 with no fragment and see if it works. If so then your question is solved. If it does not work I would start sending pings with smaller sizes till you find the one that starts to work.


HTH


Rick

anthony.dyne Wed, 09/22/2010 - 09:32
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Hi2All


Rick, i use this command


ping 192.168.1.5 -l 1500



best wishes

Anthony

anthony.dyne Wed, 09/22/2010 - 12:05
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Hi2All,


ping 192.168.1.5 -l 1500  I get reply wihtout any timeout, does this means MTU is supporting 1500



best wishes

Anthony

Correct Answer
Richard Burts Wed, 09/22/2010 - 12:09
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Anthony


I do not think that you have yet proved what is the MTU since you have not specified control in the ping about fragmentation. Try pinging this way and see what happens

ping 192.168.1.5 -l 1500 -f


HTH


Rick

anthony.dyne Wed, 09/22/2010 - 12:16
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Rick i get this as output

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.


best wishes

Anthony

Richard Burts Wed, 09/22/2010 - 12:45
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Anthony


That is proof that the MTU of the connection between the PCs is less than 1500.


HTH


Rick

Richard Burts Wed, 09/22/2010 - 13:01
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Anthony


I described how to find the size in my first response. If ping at 1500 is too big, then ping using smaller sizes till you find one that works (remembering to use the -f to specify no fragmentation). If you want to be very exact and precise then ping using 1499, then 1498, then 1497 etc till you get to a value that succeeds. Or you could make the process quicker (but slightly less precise) by ping using 1495, then 1490, than 1485 etc till you get to a value that succeeds.


HTH


Rick

anthony.dyne Wed, 09/22/2010 - 13:05
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Hi2All


Thanks a lot rick for supporting.

I got another weird issue on Error Disable, if possible to help me.

You can look at details, i posted in the forum today.


best wishes

Anthony

milan.kulik Thu, 09/23/2010 - 00:30
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Hi Rick,


a) In the MS Windows world, ping a.b.c.d -l 1472 -f  is the correct syntax to start as Windows are calculating 28 Bytes of the header length.

So ping a.b.c.d -l 1472 -f has the same result as Cisco CLI command ping a.b.c.d  size 1500 df-bit - an ICMP Echo Request packet of 1500 Bytes size sent with Don't Fragment bit sent.


b) To speed up the procedure of finding the exact maximum packet size which is passing,  you can start with 1472, then try 1372, 1272, etc.

When the first packet passes (let's say 1272), increase the size by 50. If passes, increase by another 25, if not passes, decrease by 25. Etc, etc.

This method (called interval halving in maths)  is quite effective and fast.


BR,

Milan

Richard Burts Thu, 09/23/2010 - 10:50
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Milan


I was not aware of the Windows calculation about header. Thanks for that information (and +5 for you).


I am aware of alternatives such as interval halving (I have also heard a similar technique referred to as successive approximations) and thought about suggesting that. Then I decided in the interest of simplicity to just suggest stepping through successive values. Either approach works and your approach gets the answer more quickly when the difference between where we start and the correct answer is large.


HTH


Rick

jimmysands73_2 Thu, 09/23/2010 - 00:46
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Along with all the other responses here, you can google mturoute.exe, that might be another way to get the information you wanted.

anthony.dyne Thu, 09/23/2010 - 01:15
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Hi2All


I did couple of test and found that maximum i get reply with 1270 ; does this mean my Wireless AP supports max MTU size of 1270


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ping 192.168.1.5 -l 1270 -f


Pinging 192.168.1.5 with 1270 bytes of data:
Reply from 192.168.1.5: bytes=1270 time=6ms TTL=254
Reply from 192.168.1.5: bytes=1270 time=6ms TTL=254
Reply from 192.168.1.5: bytes=1270 time=6ms TTL=254
Reply from 192.168.1.5: bytes=1270 time=7ms TTL=254


Ping statistics for 192.168.1.5:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 6ms, Maximum = 7ms, Average = 6ms


ping 192.168.1.5 -l 1275 -f


Pinging 192.168.1.5 with 1275 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.




best wishes

milan.kulik Thu, 09/23/2010 - 01:28
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Hi,


IMHO, the value can be 1271-1274, you need to try those values to get the  exact number.

If the largest packet passing is let's say 1272, the MTU on the path is 1300 Bytes (don't forget the Windows syntax).


According to the message syntax you are receiving, it could be your PC which has the MTU set to 1300 Bytes.

If the limit were somewhere further, you would usually not receive any message, the ping would simply get no reply.


BR,

Milan

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